Boiled brakes?

[poprivet]

Today was a hot day. After a normal run, nothing excessive, I was in a queue for roadworks that was all stop and start.

At one slowing down I thought Um, that was a longer brake pedal then normal. At the next one, pedal went to the floor with very minimal braking, luckily a combination of a good gap, gear selected in 2nd and the poor old handbrake yanked on nothing was hit. Quite a fright though! There were no failure lamps lit on the dash and pumping of the pedal made no difference at all.

Luckily I was very close to home, so driving like a 90 year old in a Honda Jazz, pulled onto the driveway. Oddly enough, the pedal and braking had returned to normal, I'm wondering if holding the car on the footbrake boiled the fluid?

Tomorrow I will go completely through the system, check everything, flush with new fluid and bleed it properly.

I'll let you know what I find.

[PaulGover]

In the past I've had that, but only after either (a) a track day where I miscounted the laps, so there wasn't one to cool down the brakes or (b) decending the mile or so 1 in 4 hill at Applecross - when I got to the cattle gate at the bottom, the front of the car disappeared in a cloud of steam - boiling hub grease, at a guess

Holding the car on the foot brake shouldn't cause that sort of problem; inching the car against the engine in gear for a long distance might, (especially if your idle is too high), but even so it shouldn't generate that much heat. You didn't inadvertently park over any open fires

In more normal circumstances, that sort of problem might indicate very old brake fluid that's gained a lot of moisture, which will lower its boiling point, or riding the brake pedal at speed (to generate heat roughly as (a) or (b) above). Otherwise I'd wonder about either a leaking brake pipe - I had a scary incident with that on my X350, where it felt like there was no brake pedal until I reached the last 1/2 inch of travel, where the dual circuit stuff could finally kick in. That problem temporarily vanished after pumping the pedal, as it managed to get enough fluid from the reservoir to temporarily bleed the brakes, but of course only until it too vanished through the same hole.

[ptjs1]

Pop,

Glad you got home safely! Well done!

Also consider checking if you've got a stuck caliper on one of the wheels. It's quite possible to have that and not really notice in driving or braking. A stuck caliper can definitely generate enough heat to boil the fluid. Also check for an internally collapsed flexi hose which can in turn lock one circuit and then cause the same heat generation and boil.

Good luck

Paul

[PaulGover]

...
Also consider checking if you've got a stuck caliper on one of the wheels.
...

Good point. I should have remembered that - about two years ago I parked my X350 and there was a strong smell of burning from the front LH caliper. It was radiating a lot of heat, and that was just a 2 mile trip to the shops. Both discs and pads were worn out, which is probably why the caliper seized - my guess is its piston got too far out of the cylinder and therefore could be wedged at angle. Full front brake overhaul cured it.

[poprivet]

Sorted, I hope. Discs, pads and calipers all in good condition with even pad wear. No leaks apart from a damp bleed nipple on one front caliper.

What I did find was a very low fluid level. The plastic reservoir isn't the easiest thing to make out the level marks due to slight discolouration and the join. An important thing that I had failed to realise is that as the car isn't used much, the accumulator gradually releases fluid back into the system raising the fluid level.

So when I had been checking the level to ensure it is above the min level, the system would go even lower once pressurised. In future I will make sure the level is at Max only when the accumulator pump has stopped. This means after standing for a week or so the level will appear far above the Max level but in no risk of overflowing. I wonder how many other owners have not realised this.

I would have thought the low level warning would illuminate, perhaps the float switch is faulty?

I flushed a litre of new fluid through as it was overdue although the old fluid looked fairly clear and bled the system correctly as per the manual.

Thanks for all the suggestions, like many things, we only notice these things once they go wrong.

[ptjs1]

Pop,

Glad to hear you've got it sorted. I'm still not sure how the pedal dropped so low, that seems to imply a significant amount of air in the system?

Anyway, if you've fully bled the system properly, then hopefully the problem is now resolved.

One point - Ever since I got my first XJS over 20 years ago, I was always told to bleed the Teves II ABS brakes using the procedure defined by Teves, not the one defined in the Jaguar manual. I don't know which manual you were using or the procedure documented, but below is the Teves procedure in case it's of furture help:

Start at rear left, then rear right, then front left, front right.

One person in the car, ensure brake fluid level is fully topped up.

Turn on ignition and wait until both abs and low-pressure lights have extinguished and also the motor pump has cut out.

Then, open rear left bleed nipple and hold brake pedal down, with ignition on for 15 seconds – no more, tighten bleed nipple, turn off ignition, release brake pedal.

Wait one minute and repeat from restoring system pressure as above for other nipple.

Then wait one minute at least before moving to front brakes – this prevents the motor pump from overheating.

At the front, ensure system pressure is created and then bleed calipers conventionally with ignition on, using slow strokes and go the full travel but do not force.

Once the left side has been done, again wait one minute and then restore system pressure before doing the right side.

Ensure brake fluid level is checked and topped up after each caliper is bled.


Cheers

Paul

[poprivet]

Interesting, I wonder what the difference is in practice.
I guess as long as all old fluid is flushed out and there is no air in the system there is no difference.

Could be the makings of a magazine article?

[ptjs1]

Pop,

I think the difference is in the procedure of ensuring that the pump has pressurised fully, but not letting it overheat, when the rear brakes are bled. As you know, strangely, the pedal doesn't "push" any fluid to the rear brakes. For the rear brakes only, all the pedal does is open the valve in the Actuation Unit that allows pressurised fluid from the accumulator to enter the Actuation Unit and then flows under accumulator pressure to the rear brakes. If there wasn't a proportioning valve inline, in theory the rear brakes could otherwise lock up every time you even gently touched the brake pedal.

I've been asked by a number of people to do an article to try and explain and troubleshoot the Teves II system, as it's a very unorthodox system. If you don't know the system and just tried to bleed the rear brakes conventionally, you may never get rid of the air. Maybe this is the prompt for me to write the article?

Cheers

Paul